A surgical instrument designed to measure the depth of a recess, usually to measure the depth of holes being drilled in bone so that the surgeon can determine the length of the bone screw to be used. It may take two forms: 1) a device with a moveable (sliding) centrepiece (a prong), the tip of which is inserted into the recess to read from a scale; 2) a circular disc with a central hole which is clipped to a drill bit so that the device slides up as the drill bit is advanced. This is a reusable device intended to be sterilized prior to use.